In recent years, an ink jet recording method has found wide application in various kinds of graphic art fields such as photography, various kinds of printing, marking and specific printing such as a color filter because of being able to form images easily and inexpensively. Particularly, it has also become possible to obtain image quality comparable to silver salt photograph images by utilizing a recording apparatus which ejects and controls fine dots, ink in which a color reproduction range, durability and ejection suitability have been improved, and exclusive paper in which ink absorption, color forming property of colorants and surface gloss have been greatly improved. Image quality improvement of an ink jet recording method of today has been achieved only when a complete set of a recording apparatus, ink and exclusive paper is prepared.
However, an ink jet system which requires exclusive paper is problematic in respect to limitation of a recording medium and cost up of a recording medium. Therefore, many attempts have been made which record on a recording medium different from exclusive paper, employing an ink jet recording. Concretely, there are methods such as a phase-conversion ink jet method utilizing wax which is solid at room temperature, a solvent-type ink jet method utilizing an ink which is mainly comprised of a rapid-drying organic solvent and a UV ink jet method in which an ink is cross-linked by ultraviolet (UV) light after recording.
Among them, a UV ink jet method has been noted recently in respect to odor relatively lower than that of a solvent-type ink jet method, rapid drying property and capability of recording on a recording medium having no ink absorption. UV-curable ink jet inks are disclosed, for example, in Japanese Patent O.P.I. Publication Nos. 6-200204 and 2000-504778.
However, even when these inks are employed, the dot diameter of ink ejected onto recording material changes significantly depending on the kinds of recording material or operating conditions, and it is therefore impossible to form a high resolution image on various recording materials.
In a UV-curable ink-jet ink, ink employing a cationically polymerizable compound (see for example, Japanese Patent O.P.I. Publication Nos. 2002-188025, 2000-239648, and 2002-239309, and Japanese Patent Publication No. 3437069) is not affected by oxygen, however, it has problem in that the polymerization reaction is susceptible to moisture in the molecular level (or humidity) on account of its polymerization mechanism.
A cationically polymerizable composition employing a photoinitiator generating benzene as a decomposition product on actinic ray exposure, which includes a triaryl sulfonium salt, UV16992 produced by DOW CHEMICAL CO., LTD., has been prepared (see for example, Japanese Patent O.P.I. Publication Nos. 2002-188025, 2002-239309, and 2002-239648, and Japanese Patent Publication No. 3437069). However, this composition is not applied to the field of foods on account of generation of benzene, and further has problem in that its ejection stability deteriorates depending on ambient atmosphere (such as temperature or humidity). Accordingly, it has been impossible to form an image with high precision according to an ink-jet recording method employing the composition as ink.
It is well known that at least one compound selected from a polycyclic aromatic compound having a hydroxyl group, a substituted or unsubstituted aralkyloxy group or a substituted or unsubstituted alkoxy group, a carbazole derivative and a thioxanthone derivative is used as a sensitizing agent (see for example, Japanese Patent Publication No. 3437069, and Japanese Patent O.P.l. Publication Nos. 2000-239648, and 2002-239309). However, they cannot provide stable ejection of ink-jet for ink-jet recording, and cannot practically be used.